Understanding Co-dominance and Partial Dominance Practice Test

Which of the following describes a single genetic locus that controls more than one trait?

In humans, blood type is determined by three alleles: A, B, and O. Both the A and B alleles are dominant to the O allele. When both A and B are present, however, a combined phenotype of AB results. In AB individuals, both A and B antigens are fully expressed on the cell surface. This is an example of                     .